Pre-printed (forms, etc.) or fixed size (labels, etc.) thermal media should be correctly aligned within a printer in order for the printing to properly register onto the media. The most common method to insure accurate media alignment is use of a pre-printed “black mark” on the back or the front side of the media.
Unfortunately, a front printed black mark may result in a large inter-form/label gap or may introduce an undesired element (a black mark) to a print canvas. The back printed mark also adds complexity to a printer design, since it requires installation of a reflective photoelectric sensor on the back side of the printer door. Infrared light transmitters are traditionally used for this purpose. However, traditional infrared reflective photoelectric sensors are sensitive to direct sunlight and often fail when used in a peel-and-present printing mechanism. This arrangement is also limited to only sensing a level of transmitted infrared light reflection from an object and suffers from wide variations in received signal strength due to quality and variations in the material of the sensor and its potentially imprecise placement during assembly. More often than not costly sensor sorting and matching operations have to be implemented to alleviate sensitivity error.
Therefore, a need exists for a better mechanism to detect proper alignment of forms in a printer.